1. Field of the Invention
The invention relates to an optical device for the observation of an elongated object, i.e. exhibiting dimensions much larger in one direction than in another.
2. Discussion of Background
The observation of elongated objects (or elongated zones) is a common problem. By way of example, the optical reading of magnetic tapes as described in French patent 84 08252 published under U.S. Pat. No. 2,569,072 will be cited. This patent refers to the reading on a magnetic tape of a large number of tracks, for example 1,500, all parallel to the longitudinal direction of the tape. The data contained on these tracks can be read simultaneously. If an element of data is recorded on the track over a length on the order of one micron and if the width of the tape is on the order of one centimeter, it is necessary to observe a zone 1 micron wide and 1 cm long in order to simultaneously read all of the tracks.
The observation of such elongated objects is difficult because of diffraction whereby as the field of observation of an optical system increases, its resolution decreases.
It is known that the digital opening .alpha. of an optical system and the resolution d are related by the following equation: ##EQU1## in which .lambda. is the wavelength of the light; it also is known that the larger the digital opening, the smaller the extent of the field of observation.
Thus, a good resolution was seemingly incompatible with an extended field. As an example, a resolution of about one micron when the object extends to 1 cm was very difficult to obtain.